1. Field of the Invention
The present invention relates to the field of semiconductor integrated circuit (IC) manufacturing, and more specifically, to an apparatus for and a method of removing contaminants from a mask during photolithography.
2. Discussion of Related Art
The manufacturing of IC devices involves the sequential processing of a semiconductor wafer to add or remove various layers of material. The critical layers define features in the IC design having tighter groundrules, such as those for isolation, gate, contact, and first metal while the non-critical layers define the remaining features.
Photolithography is a process of patterning each layer on the wafer. As many as 35 masks may be required to pattern all of the layers on the wafer. A mask is fabricated by depositing an opaque material, such as chrome, on a transparent or reflective substrate and then etching the chrome to form features that are 5 times larger than the desired size on the wafer.
The wafer is covered with a material called photoresist that is sensitive to light. An exposure tool, such as a stepper, shines light of a nominal wavelength and bandwidth onto the mask and optically reduces the features 5 times so as to print a latent image with the correct dimensions in the photoresist on the wafer. The photoresist is developed to create openings corresponding to the location and intensity of light exposure. Thus, the photoresist serves as a stencil for transferring the pattern, originally in the mask, into the wafer by etching or ion implanting the underlying layers through the openings.
When the minimum critical dimension (CD) of a feature on a mask becomes smaller than the exposure wavelength, the resolution of an image of the feature may be degraded by diffraction or spreading of the exposure light. A phase-shifting mask (PSM) is a type of mask that modulates the phase as well as the amplitude of the exposure light to minimize the detrimental effects of diffraction. Furthermore, optical proximity correction (OPC) may be incorporated into the mask to compensate for systematic image distortion resulting from printing or etching. The distortions may include corner rounding, line shortening, and CD offset between nested features and isolated features.
A mask, including a PSM having OPC, must be kept clean of contamination in order to ensure fidelity of the pattern transfer. A mask is kept clean by covering it with a thin and transparent material or pellicle. However, when the wavelength of the light used for exposure becomes shorter than about 180 nanometers (nm), the vicinity of the mask must be purged to keep away any contaminant that may absorb light at the exposure wavelength.
Thus, what is needed is an apparatus for and a method of keeping contaminants away from a mask during exposure in a stepper.